United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 94-46: Nonconservative Reactor Coolant System Leakage Calculation


June 20, 1994



All holders of operating licenses or construction permits for nuclear power


The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to the potential for water from sources other than
the reactor to be routed to closed system tanks inside the containment and
thus cause a nonconservative evaluation of unidentified reactor coolant system
leakage.  It is expected that recipients will review the information for
applicability to their facilities and consider actions, as appropriate, to
avoid similar problems.  However, suggestions contained in this information
notice are not NRC requirements; therefore, no specific action or written
response is required.

Description of Circumstances

McGuire Nuclear Station.  On December 7, 1993, during review of the leakage
inputs into the reactor coolant drain tank, the licensee, Duke Power Company,
determined that several valve stem leakoff lines other than those associated
with the reactor coolant system or the chemical and volume control system were
connected to the tank.  In particular, leakoff water from the cold leg
accumulators and the refueling water storage tank was collected in the reactor
coolant drain tank.  The connected cold leg accumulator leakoff lines included
the four nominal 10 in. discharge isolation valves, four nominal 1 in. fill
isolation valves, and seventeen nominal  in. or 1 in. isolation valves
associated with the test header for cold leg accumulator and safety injection
check valves.  In addition, a discharge line from a relief valve on a flush
line to the regenerative heat exchanger was connected to the reactor coolant
drain tank.  Leakoff from these sources incorrectly increased the value of
identified RCS leakage, resulting in a nonconservative value of unidentified
leakage from the reactor coolant system (Licensee Event Report (LER)

Upon discovery, the licensee conservatively labeled the measured total reactor
coolant system leakage as unidentified.  At that time, total leakage was
calculated to be less than 3.8 l/min [1 gal/min]; therefore, the technical
specification limit of 3.8 l/min [1 gal/min] for unidentified leakage was not
exceeded.  At each unit, the licensee redirected these leakoff lines to the
containment sump and reevaluated unidentified leakage for the current

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June 20, 1994                                         Page 2 OF 3

operating cycle.  The licensee found the unidentified leakage to be no greater
than 4.9 l/min [1.3 gal/min] greater than previously evaluated.  The licensee
concluded that some leakoff from the non-reactor sources had drained into the
reactor coolant drain tank at each unit and that the technical specification
limit for unidentified leakage had been exceeded several times during the
current operating cycle for Unit 2.

Catawba Nuclear Station.  In a similar review, the licensee, also Duke Power
Company, determined that the leakoff lines from the four nominal 10-in. cold
leg accumulator discharge isolation valves were connected to the reactor
coolant drain tank.  The licensee redirected these lines to the containment
sump, reevaluated operability, and determined that the technical specification
unidentified leakage limit had not been exceeded during the current operating


In a pressurized water reactor (PWR), reactor coolant system leakage inside
containment is typically classified as "identified" or "unidentified."
Identified leakage includes the coolant leakage directed into closed system
tanks, such as the leakoff systems for the reactor coolant pumps or valves
routed to the reactor coolant drain tank and the leakoff systems for
pressurizer safety and relief valves routed to the pressurizer relief tank.
All other leakage is labeled "unidentified."

The licensee assigned the cause of the McGuire event to failure in the
original design of the computer program for calculating reactor coolant system
leakage to recognize inputs of water from other systems to the reactor coolant
drain tank.  Most PWR leakage detection calculations use measurements of
changes in the reactor coolant system inventory over a specified time period.
The total leakage is obtained from the sum of subsidiary inventory changes,
such as changes in the inventories of the volume control tank and of the
pressurizer.  The identified leakage component is determined from changes in
the closed system inventory, and the unidentified component is then determined
by subtracting the identified leakage from the total leakage.  Typical
technical specifications for PWRs, as is the case here, set the limiting
condition for operation for unidentified leakage at 3.8 l/min [1 gal/min] and
stipulate periodic determinations of this leakage..
IN 94-46                                         June 20, 1994
                                        Page 3 of 3

This information notice requires no specific action or written response.  If
you have any questions about the information in this notice, please contact
one of the technical contacts listed below or the appropriate Office of
Nuclear Reactor Regulation project manager.


Brian K. Grimes, Director
Division of Operating Reactor Support
                    Office of Nuclear Reactor Regulation

Technical contacts:  George F. Maxwell, RII
   (704) 875-1681

               John Zeiler, RII
   (803) 831-2963

               Garry A. Harris, RII
               (704) 875-1682

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